Bracken fern (BF) is a food delicacy consumed by humans and serves as an incidental or obligatory animal forage in many areas of world. Human BF consumption in Japan was correlated with an increased incidence of esophageal carcinomas. Cows exposed naturally or deliberately to BF developed urinary bladder carcinomas within 2-5 years. Rats, mice and guinea pigs were equally susceptible to the carcinogenic effects of BF. BF induced simultaneous bladder and intestinal carcinomas in these species. BF carcinogenicity was enhanced or inhibited by dietary supplementation of various chemicals. Lactating cows fed BF excreted in their milk an intestinal and urinary tract carcinogen for rats and a mutagen for S. typhimurium TA 100. This finding suggested that milk from cows exposed to BF may provide a source of BF carcinogen for humans. We isolated and identified several flavonoid compounds, namely isoquercetin, astragalin, and tannin, and some organic acids. Oral administration of tannin from BF to rats produced neither intestinal nor bladder tumors. However, histiocytomas were induced in 80% rats by sc injections of BF tannin. We demonstrated that quercetin and kaempferol obtained by hydrolysis of isoquercetin and astragglin, respectively, were mutagenic in S. typhimurium TA 98 and TA 100. We found that quercetin was carcinogenic for the intestinal and bladder epithelium of rats. Quercetin represents a carcinogen of a new structural class. It is ingested in significant quantities by human consumption of vegetables, fruits, tea and BF which are the main sources of conjugated or free form of quercetin. However, there is another mutagenically active compound (not kaempferol or quercetin) in BF. Identification of this compound is in progress.